1. Field of the Invention
The present invention relates to a small DC motor, and more particularly to fixing magnetized position setting means for a permanent magnet and means for fixing the permanent magnet to a case constituting a motor case.
2. Related Art
In a small DC motor 100 such as that shown in FIG. 9, usually, an annular permanent magnet 102 which is produced by sinter forming is attached to the inside of a cup-like motor case 101, and the outer peripheral face of an armature 106 which is rotated integrally with a rotary shaft 105 supported on a bearing 104 is opposed to the inner wall face 103 of the annular permanent magnet 102 with maintaining a predetermined gap d. A commutator unit 108 having a plurality of slits in the circumferential direction is fitted onto the rotary shaft 105. A pair of brushes 109 are slidingly contacted with the commutator unit 108, so that coils 107 of the armature 106 are energized via the brushes 109 and the commutator unit 108. In the small brush DC motor, the switching accuracy of the energization of the coils, i.e., that of the polarity switching depends on relative positional relationships among the magnetized positions of the permanent magnet 102 with respect to the motor case 101, the positions of the slits of the commutator unit 108 with respect to the salient poles of the armature core 106, the positions where the brushes 109 are slidingly contacted with the commutator unit 108, and the position where the brushes 109 are fixed with respect to the motor case 101 to which the permanent magnet 102 is attached. When the positional relationships are not matched with each other or any one of the positions is deviated, torque ripple and cogging are increased, thereby impairing the characteristics of the motor. The phase number of the motor, i.e., the number of operations of switching the polarity may be increased in order to increase the torque of a motor, reduce the power consumption, and reduce the size of a motor. In such a case, particularly, the impairment of the motor characteristics due to such positional relationships becomes remarkable.
Therefore, means for enhancing the positional accuracy of the permanent magnet with respect to the motor magnet is considered.
In the small DC motor 100 of the prior art, usually, means for positioning and fixing the annular permanent magnet 102 to the inside of the cup-like motor case 101 is realized by bonding means using an adhesive agent, or integral molding means for outsert molding the annular permanent magnet and the cup-like case.
In the case of the bonding means using an adhesive agent, however, there is no reference for positioning, and the positional accuracy between the magnetized positions and the fixing position in the case becomes unstable. Furthermore, there arise disadvantages that the bonding strength is not stabilized because of the application thickness of the adhesive agent, uneven application, the ambient temperature, the curing time, and the like, and that the adhesive agent may be applied to an undesirable place.
By contrast, in the case of the integral molding means by outsert molding, the positional accuracy between the magnetized positions and the case can be sufficiently ensured by performing integral molding before magnetization. However, the cup-like case has the bottom, and hence the bottom obstructs the magnetization, so that the total magnetization is hardly performed. Furthermore, the integral molding means involves dispersion of magnetization due to misalignment, and hence the rotation torque characteristics of the motor are impaired. When integral molding is performed after magnetization, the molding temperature causes the magnetizing force to be impaired.